1. Field of the Invention
The present invention relates to an airflow pulverization and classification device for forming a particulate material. In addition, the present invention also relates to a pulverization method.
2. Discussion of the Background
Toner is typically used for developing an electrostatic latent image in image forming methods such as electrophotography and electrostatic recording methods. In order to produce high quality images (such as high definition images), toner having a relatively small particle diameter is preferably used. Such a small toner can be prepared by a pulverization method having the following processes:
(1) Heating and kneading toner constituents such as binder resins and colorants (e.g., dyes, pigments and magnetic materials) to prepare a toner constituent mixture;
(2) Cooling the kneaded toner constituent mixture to solidify the toner constituent mixture; and
(3) Pulverizing the solidified toner constituent mixture, followed by classification to prepare toner particles having a desired average particle diameter.
In the pulverization and classification process, an airflow pulverization and classification device including a pulverizer in which a toner constituent mixture is accelerated and collided with a collision plate by jet stream to be pulverized, and a classifier which works with the pulverizer and in which the pulverized particles are classified using swirling flow formed at an upper portion of the pulverizer. Specific examples of the pulverization/classification device include an impact dispersion separator (IDS from Nippon Pneumatic Mfg. Co., Ltd.), which is illustrated in FIG. 1.
The pulverization/classification device illustrated in FIG. 1 is mainly constituted of a classifier 7, a coarse particle receiving chamber 8 and a pulverizer 9. The operation of the classifier 7 is as follows. A powder to be classified, which is supplied from a hopper 1 of the classifier 7, is fed to an entrance 2a. The powder passing the entrance 2a is dispersed in a dispersion chamber 2. Specifically, the powder is swirled so as to form counter free vortex by secondary airflow 2b supplied from the outside into the dispersion chamber 2, thereby classifying the powder into relatively fine particles and relatively coarse particles utilizing difference of the centrifugal force and centripetal force applied to the particles of the powder. The fine particles, for which a further pulverization operation is not necessary, are fed so as to be subjected to the next process. In contrast, the coarse particles fall into the coarse particle receiving chamber 8 by their own weight, and the coarse particles then enter into the pulverizer 9 through a casing hopper 3 serving as a feeder. In the pulverizer 9, coarse particles 10 sucked from an entrance 4 are collided with a collision plate 6 in a pulverization chamber 11 by jet flow 13 supplied from a jet nozzle 12 by an airflow source 13a (such as compressors) after being accelerated by a pulverization nozzle 5. The thus pulverized coarse particles are fed again to the dispersion chamber 2 together with the powder supplied from the hopper 1. Thus, the powder is subjected to a closed circuit pulverization and classification operation. Referring to FIG. 1, numerals 14, 15 and 16 respectively denote an acceleration tube, a supply tube, and a junction between the acceleration tube 14 and the supply tube 15.
Recently, a need exists for color image forming apparatus which can produce high quality color images at a high speed. In order to fulfill the need, the toner used for such color image forming apparatus is required to have a low melting point and a small average particle diameter. When such a toner is prepared using an airflow pulverization and classification device, problems in that the pulverized toner particles are adhered to the parts and inside walls of the device; and the pulverized toner particles aggregate, resulting in formation of coarse particles tend to be caused. In conventional airflow pulverization and classification devices, the conditions of the pulverized particles in the closed circuit of the device cannot be recognized, and thereby problems in that the pulverized particles aggregate with time, resulting in formation of coarse particles; and the pulverization chamber is clogged with the aggregated particles tend to be caused. In this case, the pulverization and classification operation of the devices has to be stopped, resulting in deterioration of the manufacturing efficiency and the yield of the toner.
When an external force applying device such as air vibrators, knockers and bridge breakers is used for preventing occurrence of the toner adhesion and aggregation problems, other problems such that the working conditions are worsened due to the noise and vibration caused by the device; and the metal parts constituting the pulverization/classification device are cracked due to the impact and vibration of the external force applying device, resulting in breakage of the pulverization/classification device tend to be caused.
In attempting to solve the problems, various airflow pulverization/classification devices have been proposed. For example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 05-309288, which corresponds to U.S. Pat. Nos. 5,577,670 and 5,839,670, discloses a fine powder production device, in which a high pressure gas jet nozzle is set so as to extend in the vertical direction in attempting to efficiently perform pulverization while preventing occurrence of the toner adhesion and aggregation problems, and preventing local abrasion of the parts of the device (such as collision member and acceleration tube) caused by collision of the particles.
JP-A 05-15801, which also corresponds to U.S. Pat. Nos. 5,577,670 and 5,839,670, discloses a fine powder production device in attempting to prevent occurrence of the toner adhesion and aggregation problems and to prevent local abrasion of the parts of the device (such as collision member and acceleration tube) caused by collision of the particles.
JP-A 08-299833 corresponding to U.S. Pat. No. 5,765,766 discloses a jet mill in which a raw material to be pulverized is collided with a collision plate without excessively decelerating the speed of the gas by preventing formation of shock wave on a downstream side from the entrance of the raw material to be pulverized to enhance the pulverizability of the mill.
JP-A 07-136543 discloses a pulverizer in which an injection portion of a supply tube for feeding a raw material to be pulverized is set so as to be relatively slanted in the direction toward the exit of the acceleration tube compared to the guide portion, through which the raw material is supplied, to smoothly feed the material in the acceleration tube while increasing the speed of the raw material fed in the acceleration tube, resulting in prevention of clogging of the supply tube with the raw material.
JP-A 09-29127 discloses a pulverizer in which a jet nozzle is set so as to extend in the vertical direction and a raw material to be pulverized is supplied to the jet nozzle so as not to be far apart from the center of the acceleration tube of the jet nozzle in attempting to improve the pulverization efficiency while miniaturizing the pulverizer.
JP-As 09-206621 and 08-52376 have disclosed collision type airflow pulverizers in which after a high pressure gas in the accelerating nozzle is supersonically accelerated, the speed of the gas is maintained in the nozzle to collide a raw material to be pulverized against a collision plate while dispersing the material, resulting in enhancement of the pulverization efficiency.
As mentioned above, a need exists for a pulverization/classification device which can stably perform pulverization without causing the problems in that the pulverizer is clogged with aggregated particles and particles adhered to the pulverizer.